1. Field of the Invention
This invention relates to an improved frequency synthesizer and more particularly to an improved digital frequency synthesizer.
2. Prior Art
The frequency generating section of a transceiver typically includes a frequency synthesizer having a phase locked loop adapted to generate one frequency for each of many channels selectable by a front panel control when a "transmit" mode is selected and another frequency set off by the intermediate frequency (IF) when a "receive mode" is selected. Digitally programmed frequency synthesizer is often used for this purpose as a local oscillator in the channelized transceiver. With the present state of technology and available parts, the implementation of a synthesizer to generate a band of channelized frequencies is relatively straight forward and typically the voltage controlled oscillator (VCO) and loop filter are left as the design variables in the implementation. The synthesizer allows the channel selection by the use of a front panel control where switches or some other suitable means, such as thumb wheel switches, are provided to control the selection of the transmit frequency.
Several approaches exist for obtaining the frequency offset required during the receive mode according to the current state of the art: They are, mixing, direct logic implementation and adders. The mixing approach is used extensively because of its relatively low cost and simplicity. However, the mixing technique has the disadvantage of requiring an additional oscillator for the IF frequency. Moreover, except for the applications requiring a narrow tuning range, a tank circuit required for the mixer must be designed to track the synthesizer oscillator. Also it has been found that the mixing technique tends to generate spurious noise frequencies which can severely degrade the synthesizer system performance.
For certain applications where tuning range is wide and the IF frequency is simple (e.g., 10 MHz, 20 MHz, etc), the direct logic implementation approach is used to the programming inputs to add, on command, the IF frequency to the transmit frequency to provide the receive frequency. This technique is used in some aircraft radios built today. However, it becomes prohibitively expensive, because of the logic complexity, large number of circuit packages and the high power required for the standard IF frequencies, such as, 10.7 MHz, 21.4 MHz, etc. and for frequency bands where the IF offset cannot be accomplished by adding or subtracting a number to the most significant divider in the divider chain.
The adders offer a universal solution for frequency offsetting for any IF frequency but are used in very few applications because of the large number of complex logic circuit packages required, large power dissipation, high price and difficulty of obtaining natural binary coded decimal adders (NBCD).